This invention relates to armored wellbore logging cables. In some aspects, the invention relates to high strength cables based upon corrosion resistant stranded wire strength members used with devices to analyze geologic formations adjacent a wellbore, or to perform mechanical services in wellbores.
Generally, geologic formations within the earth that contain oil and/or petroleum gas have properties that may be linked with the ability of the formations to contain such products. For example, formations that contain oil or petroleum gas have higher electrical resistivity than those that contain water. Formations generally comprising sandstone or limestone may contain oil or petroleum gas. Formations generally comprising shale, which may also encapsulate oil-bearing formations, may have porosities much greater than that of sandstone or limestone, but, because the grain size of shale is very small, it may be very difficult to remove the oil or gas trapped therein. Accordingly, it may be desirable to measure various characteristics of the geologic formations adjacent to a well to help in determining the location of an oil- and/or petroleum gas-bearing formation as well as the amount of oil and/or petroleum gas trapped within the formation.
Tools, such as logging or mechanical services tools, may be lowered into the well to make measurements or perform tasks at different depths along the well. These tools may include gamma-ray emitters/receivers, caliper devices, resistivity-measuring devices, neutron emitters/receivers, and the like, which are used to sense characteristics of the formations adjacent the well. An armored cable, with or without electrical insulated conductors, connects the tool with surface equipment, and/or connects a plurality of tools together, for such purposes as transmitting electrical power, transmitting data, and/or providing structural support to the tools as they are moved through the wellbore. Generally, the cable is spooled out of a drum unit from a truck or an offshore set up, over a few pulleys, and down into the well. Armored cables must often have high strength to suspend the weight of the tool(s) and cable length itself.
Wireline cables are typically formed from a combination of metallic conductors, insulative material, filler materials, jackets, and armor wires. The jackets usually encase a cable core, in which the core contains metallic conductors, insulative material, filler materials, and the like. Armor wires usually surround the jackets and core. The armor wires used in wireline cables serve several purposes. They provide physical protection to the conductors in the cable core as the cable is abraded over downhole surfaces. They carry the weight of the tool string and the thousands of feet of cable hanging in the well. Two common causes of wireline cable damage are armor wire corrosion and torque imbalance. Corrosion commonly leads to weakened or broken armor wires.
Armor wire is typically constructed of cold-drawn pearlitic steel coated with zinc for corrosion protection. While zinc protects the steel at moderate temperatures, studies have shown that passivation of zinc in water (that is, loss of its corrosion-protection properties) can occur at elevated temperatures. Once the armor wire begins to rust, it loses strength and ductility quickly. Although the cable core may still be functional, it is not economically feasible to replace the armor wire, and the entire cable must be discarded. Once corrosive fluids infiltrate into the annular gaps, it is difficult or impossible to completely remove them. Even after the cable is cleaned, the corrosive fluids remain in the annular spaces damaging the cable. As a result, cable corrosion is essentially a continuous process beginning with the wireline cable's first trip into the well.
When an axial load is applied onto a cable, the helical arrangement of the armor wire causes the cable to develop a torsional load. The magnitude of this load depends on the helix arrangement and the size of the armor wires. There are two traditional ways of reducing the magnitude of torque that is developed: (1) increase the helix length substantially, or (2) use lower diameter armor wires on the outside and higher diameter on the inside. Neither of these options is very practical with wireline cable. The first approach increases the rigidity of the cable to flexure. The second approach may lead to decreased cable life due to abrasion issues. The cable also experiences reduction in the diameter due to the radial forces that develop during cable loading. This compresses the cable core and can cause insulation creep on conductors, leading to possible short circuits or broken conductors. During torsional loading of the cable, the effective break load of the cable will decrease due to a change in the load distribution over the two layers of armor wires. Also, when inner and outer wire armor layers, each having wires orientated in helix configurations, are used, this leads to torque development when the cable is placed under an axial load.
Another problem encountered with traditional armored wire cables occurs in high-pressure wells, the wireline is run through one or several lengths of piping packed with grease to seal the gas pressure in the well while allowing the wireline to travel in and out of the well. Because the armor wire layers have unfilled annular gaps, gas from the well can migrate into and travel through these gaps upward toward lower pressure. This gas tends to be held in place as the wireline travels through the grease-packed piping. As the wireline goes over the upper sheave at the top of the piping, the armor wires tend to spread apart slightly and the pressurized gas is released, where it becomes an explosion hazard.
Thus, a need exists for high strength armored wellbore electric cables that have improved corrosion resistance and torque balancing, while being efficiently manufactured. Further, a need exists for cables which help prevent or minimize gas migration from a wellbore. An electrical cable that can overcome one or more of the problems detailed above while conducting larger amounts of power with significant data signal transmission capability would be highly desirable, and the need is met at least in part by the following invention.